In 3 active transmission areas LaCrosse virus (LAC) (California group) survived the winter in Aedes triseriatus eggs in at least 50 percent of adequately sampled treeholes. Overall, approximately 0.4 percent of the overwintering eggs were found infected. This indicates that closure of a high percentage of basal treeholes in an area may result in interruption of transmission. An experiment to determine the incidence of transovarial transmission of LAC virus in a laboratory strain of Aedes triseriatus revealed 68 percent infection of progeny from infected females. Transgeneration persistence of transovarial infection has, at present, persisted through the F2 generation. In the laboratory the mean transmission rate of LAC virus by an endemic strain of A. triseriatus at dosage levels from 3.8 to 5.5 SMLD 50/.003 ml was 67 percent, while the mean transmission rate by a mosquito strain from outside the endemic area was only 42 percent, giving preliminary indication that transmission efficiency of various A. triseriatus populations may be an important factor in determining LAC virus endemicity. As part of a study of a possible competitive relationship between Aedes henderson and A. triseriatus, A. hendersoni was found to be the more arboreal of the two specimens with 50 percent of its oviposition activity at 10-30 ft above ground level, while 80 percent of A. triseriatus oviposition was at ground level.